FIGS. 1 through 5 depict a prior art variable reluctance position sensor 10, having three major components: a spool assembly 12, a harness assembly 14, and a cap 16 which scalingly encloses the spool and harness assemblies. Additionally, an O-ring 18 and a bracket 20 provide an installation interface for the cap 16.
The spool assembly 12 includes a reluctance sensor 24 including a magnet 24a, a (low carbon steel) pole piece 24b and a magnet wire coil 24c wrapped around the pole piece. The reluctance sensor 24 senses magnetic field variation due to movement of an adjacent ferromagnetic article, as for example rotational motion of a toothed target wheel. The harness assembly 14 includes a harness body 26 connected with a wiring harness 28 which provides remote connection to an external circuit. The spool assembly 12 further includes an upper disk 30 which supports a pair of spool electrodes 32, as well as a pair of peripherally disposed and diametrically arranged engagement barbs 34.
The harness body 26 has an annular recess which carries a resilient annular seal 38. At the upper end of the harness body is an overhanging annular lip 40 out of the center of which projects, in sealing relation, wiring 42 of the wiring harness 28. At the lower end of the harness body 26 are a pair peripherally disposed, and diametrically arranged, engagement clips 44 (only one being visible in the views). Also at the lower end of the harness body 26 are located a pair of harness electrodes 46 which are electrically connected with the wiring harness 28.
The cap 16 has a cup-shaped sidewall 48, including a bottom 50. The sidewall 48 defines a cavity 52 which communicates with an opening 54.
The spool assembly 12 is mated to the harness assembly 14 by the engagement barbs 34 being mechanically snapped into the engagement clips 44. When so mechanically joined, each spool electrode adjoins a respective harness electrode to thereby provide a pair of adjoined electrode pairs 36. Now each adjoined electrode pair is resistance welded together to provide a good electrical connection therebetween so as to provide a second mechanical joinder between the spool and harness assemblies.
The spool and harness assemblies 12, 14 are then received into the cavity 52 through the opening 54, the spool assembly being first inserted. When the spool assembly bottoms out at the bottom 50 of the cap 16, insertion is complete, wherein the annular seal 38 seals with respect to the sidewall 48 and the annular lip 40 protectively encircles the opening 54. Next, the sidewall 48 is locally heated using a shaped blade and pushed into a couple of grooves located on the harness assembly in order to stake 35 the harness assembly to the cap. Additionally, the harness assembly 16 has a nib 56 protruding from its side which fits into a square cut-out 58 located on the top edge of the cap, wherein the fitting of the nib into the cut-out serves as the anti-rotation feature of the harness assembly relative to the cap.
This type of sensor produces an analog output. The magnitude of the output is directly proportional to the number of turns of wire that make up the coil multiplied by the change in magnetic flux per unit time. The change in magnetic flux per unit time is a function of the target wheel speed. The faster the target rotates the larger the amplitude of the output voltage. If a large output is required at a low target speed the output amplitude is increased by increasing the number of turns of wire in the coil. Variable reluctance position sensors have many uses, for example to detect the speed of rotating shafts in automotive applications.
In operation, although the spool and harness assemblies 12, 14 are mechanically joined, firstly at the clips/barbs 44, 34 and secondly at the electrode pairs 36, and although the harness assembly is staked 35 to the cap 16 and the spool assembly bottomed out on the bottom 50 of the cap, there yet remains a problem associated with vibration fatigue occasioned by a lack of affixment of the spool assembly relative to the cap adjacent the upper disk 30. In this regard, the staking and bottoming out will eliminate motion between the components in the axial direction, but since a separation clearance S (see FIG. 4) must exist between the spool and harness assembly vis-a-vis the inside surface of the sidewall 48 of the cap (for assembly purposes), vibration of the sensor 10 can cause the spool assembly to move radially within the limits of the separation clearance at the periphery 30p of the upper disk 30 with respect to the with the cap. This vibration induced radial movements results in stress at the mechanical joints (electrical and mechanical) between the terminal pairs 36, possibly over time leading to a failure. In this regard, this stress will be greater for (axially) longer spool assemblies.
Accordingly, what is needed in the art is to somehow reduce the stress the mechanical joints experience in a variable reluctance rotary position sensor.